spark

impetus - noun
1. An impelling force; an impulse.
2. The force or energy associated with a moving body.
3. a.Something that incites; a stimulus.
b.Increased activity in response to a stimulus: The approaching deadline gave impetus to the investigation.

catalyst –noun
1. Chemistry. a substance that causes or accelerates a chemical reaction without itself being affected.
2. something that causes activity between two or more persons or forces without itself being affected.
3. a person or thing that precipitates an event or change: His imprisonment by the government served as the catalyst that helped transform social unrest into revolution.
4. a person whose talk, enthusiasm, or energy causes others to be more friendly, enthusiastic, or energetic.